This invention relates to shovel type excavators, and more particularly to front end mechanisms for use in these and similar machines.
Shovel type excavators for construction, mining and other earth moving tasks are characterized by the provision of a forwardly extending boom to which a dipper stick is pivotally attached. A dipper, or bucket is mounted on the outer end of the dipper stick, and digging is accomplished by driving the dipper through the earth in a shoveling or scooping movement. The loaded dipper is then raised and swung to a position where the load is dumped. Usually, an excavator imparts two principal movements to its dipper. One is a crowdretract motion in which the dipper is translated either outward or inward of the machine proper. The second is a hoist-lower motion in which, as the term implies, the dipper is either raised or lowered. When the two motions are coordinated the dipper can be manipulated in most any path desired within the envelope of its permissible range of movement.
Conventional type power shovels used for mining and excavating employ a boom fixed in a position extending obliquely forward and upward from a machinery platform. A dipper stick is pivotally mounted approximately at the longitudinal center of the boom and is also reciprocably movable in its lengthwise direction. A dipper is mounted at the outer end of the dipper stick, and a hoist rope runs upwardly from the dipper to a sheave at the top of the boom, from which it extends downwardly to the machinery platform. This form of construction is illustrated in U.S. Pat. No. 3,690,483 entitled "Power Shovel Front End". Hoist-lower motion for such a shovel is imparted to the dipper by drawing in and paying out the hoist rope. The crowd-retract motion for the dipper is accomplished by thrusting the dipper stick outwardly along its axial length for crowding, and by drawing it inwardly for retracting. The dipper is usually fixed in position at the outer end of the dipper stick, so the dipper teeth remain at all times in a set orientation with respect to the dipper stick. The hoist motion supplies the primary digging force, and normally pulls the dipper upwardly through a bank being excavated. The crowd motion is also applied to the dipper during the digging stroke to urge the dipper into the bank being excavated. The crowding force, however, contributes little to the digging force exerted at the dipper teeth, and the resulting path for the dipper through a complete digging stroke takes a generally parabolic configuration entailing a considerable upward component of motion.
One drawback to the digging pattern of a conventional type power shovel is that the digging force applied through the upward pull of the hoist rope is oblique to the path of excavation traveled by the dipper teeth. As a result, the digging force available is not applied in the most efficient manner. Further, the crowd force required to keep the dipper in the bank pushes the sides of the dipper teeth and the dipper heel into the earth, so that considerable sliding friction is created between the dipper and the material being excavated. This friction must be overcome by the hoist rope pulling the dipper through the bank, with a resultant inefficiency.
Another deficiency of the conventional type shovel is that due to the large hoist component in the digging stroke the ability to cut a flat floor over an area is extremely limited. In mining operations, after excavation is completed it is frequently necessary to use other forms of machinery for cleaning up the area and leaving a level surface. A still further problem of these shovels concerns shallow excavations. Since substantial vertical cuts must be taken to fill the dipper, inefficiency results when the bank being excavated does not afford adequate vertical height for taking a full cut.
Another form of common construction for large power shovels is shown in U.S. Pat. No. 3,376,983 entitled "Rope Crowd for a Knee Action Shovel". In this construction, the inner end of the dipper stick is pivoted to the upper end of the boom to provide an articulated, knee-type action between the boom and dipper stick. A separate crowd arm is also connected to the boom upper end for moving both the boom and dipper stick outward and inward in a crowd-retract stroke. The hoist motion is similar to the conventional type of construction discussed above, with a hoist rope extending downward from a sheave at the top of a mast to the dipper. The digging motions of these shovels are comparable to that of the conventional type machines already discussed, and they exhibit similar problems.
In recent years there has been a development of hydraulically operated mining and excavating shovels using hydraulic cylinders for manipulating a boom and dipper stick, and two forms of such shovels are shown in U.S. Pat. No. 3,491,906 for "Loader Apparatus with Crowd Capability" and U.S. Pat. No. 3,578,188 for "Bucket Actuated Means for Excavator". These excavators take the form of enlarged scooper-loaders, with the dipper stick pivoted at its inner end to the forward end of the boom. The boom pivots about its foot, or lower end, so that both the dipper stick and boom are pivoted in manipulating the dipper. One hydraulic cylinder pivots the boom, and is usually attached to the machinery deck. A second hydraulic cylinder extends outwardly to a connection with the dipper stick to pivot the dipper stick relative to the boom. Hydraulic cylinder components have been limited as to their practical size, and they also require considerable maintenance. Hydraulic cylinders and lines connected thereto are susceptible to dirt, dust and other debris, and when located in exposed positions near the point of digging are subject to damage. It has, therefore, remained desirable to employ a different manner of drive for an excavator to eliminate these deficiencies.
Electrical drive systems have been used in the conventional type of mining excavators which are highly reliable in their operation. They efficiently and effectively operate for long periods of time with less maintenance, and consequently are more desirable than the hydraulic cylinder type drive systems. Electric drives have, however, been limited in application to conventional front end designs having the drawbacks discussed above. It would be desirable to combine electric drives with the boom-dipper stick designs similar to those of hydraulic shovels, and to also incorporate a wristing motion for the dipper comprising a pivot of the dipper about its connection with the dipper stick. A wristing motion can improve machine versatility in loading and dumping, but has not been used to any substantial extent in conventional type electric drive shovels. Wristing is used extensively in the hydraulic machines referred to above, and has been taught in a limited fashion for use in front end systems of conventional mining machines, as in U.S. Pat. No. 3,243,063 entitled "Variable Pitch Excavator Dipper Mounting". It would be desirable to combine a full wristing action with an electric drive that powers the hoist, crowd and swing motions of the excavator.